Radiolabelled phenylethyl imidazole carboxylic acid ester derivatives

ABSTRACT

Halogenated carboxylic ester derivatives of phenylethyl imidazole, and their method of preparation are disclosed. Radio-halogenated forms of these compounds are ideally suited for positron-imaging of the adrenal glands, as it is known that these compounds demonstrate a selective and high rate of accumulation in the adrenals. The method of preparing these derivatives proceeds by the conversion of a stable, non-radioactive intermediate having trialkylstannyl leaving groups. These intermediates are efficiently converted to the corresponding halogenated forms by substitution of the trialkylstannyl group with the halogen or radiohalogen.

SUMMARY OF INVENTION

The invention relates to radioactively labelled derivatives of(R)-3-(1-phenylethyl)-3H-imidazole-4-carboxylic acid esters and methodsfor preparing these compounds. The invention also relates to the use ofthese radioactively labelled compounds as radiopharmaceuticals. Inparticular, these compounds bind selectively to adrenocortical tissuefacilitating the diagnosis of adrenal cortical masses such asincidentaloma, adenoma, primary and metastatic cortical carcinoma.

BACKGROUND OF INVENTION

The present invention relates to a class of substituted(R)-3-(1-phenylethyl)-3H-imidazole-4-carboxylic acid esters whichinteract selectively with the mitochondrial cytochrome P-450 species inthe adrenal cortex (Vanden Bossche, 1984). When labelled withradiohalogen (iodine-123; bromine-76; fluorine-18 and otherrs) thesecompounds serve as radiotracers for the diagnosis of adrenal corticalmasses such as incidentalomas, adenomas, primary and metastatic corticalcarcinoma. When labelled with a beta-emitting radionuclide (iodine-131;bromine-82), these radiotracers may be used for radionuclide therapy.The main application is for tumour diagnosis (Khan 2003).

In particular, the compounds according to this invention are potentinhibitors of steroid P450_(β)hydroxylation and bind with high affinityto adrenocortical membranes. In fact, the compounds in accordance withthis invention have been found to possess an almost 1000-fold selectiveaffinity when compared with known, clinically used inhibitors(metyrapone, ketoconazole). Therefore, when injected intravenously, thelabelled derivatives of the present invention, accumulate rapidly in theadrenals, reaching radioactivity levels that are diagnostically useful.

The parent compound etomidate (ethyl ester; ETO) is clinically used as ashort-acting hypnotic drug. When incubated with human adrenocorticaltissue slices, it was shown to block the conversion of 11-deoxycortisolto cortisol and of 11-deoxycorticosterone (DOC) to corticosterone andaldosterone (Weber 1993; Engelhardt 1994). Also metomidate (MTO), themethyl ester, is an equally potent inhibitor of steroid11β-hydroxylation. (R)-configuration of the methyl substituent at thechiral C-atom is essential for enzyme inhibition (Vanden Bossche, 1984).

Clinical findings with the radiotracer [O-methyl-¹¹C]metomidate haveindicated high uptake in lesions of adrenocortical origin, includingadenomas, but very low uptake in lesions of non-adrenocortical origin(Bergström 1998; 2000). Specific uptake has been reported in multiplemetastases in the lung of a primary adrenocortical carcinoma(Mitterhauser 2002). However, the differentiation between benign (e.g.,adenoma) and malignant (e.g., carcinoma) is primarily based on the sizeand shape of the lesion; irregularities in tumour uptake and multiplelesions are an indication of malignancy (Khan 2003).

Although ¹¹C-metomidate has “ideal” biological characteristics forscintigraphy of the adrenals and tumor derived therefrom, application ofthe radiopharmaceutical is limited to hospitals with a PET facility. ¹¹Cis a cyclotron product and decays with a half-life of 20 min, therefore,¹¹C-metomidate must be synthesized immediately prior to use.

Halogenations, on the other hand, offer sufficient flexibility, time forpreparation and shipment. (Iodine-123 T_(1/2)=13.2 hours; Br-76T_(1/2)=16 hours).

Enzyme inhibitors, such as metyrapone have been labelled withradioiodine for adrenal scintigraphy, however, these compounds havenever been used for clinical diagnosis (Wieland, 1982; Robien & Zolle,1983). A comparison of the binding affinities (IC₅₀-values) of knowninhibitors with etomidate clearly demonstrated the higher potency ofetomidate and metomidate.

The available radiotracers for imaging the adrenal cortex and adrenalcortex-derived tumors are labelled cholesterol derivatives. Theseinclude 6β-[¹³¹I]-iodomethyl-19-norcholesterol (NP-59) (Basmadjian,1975) and 6β-[⁷⁵Se]-selenomethyl-19-norcholesterol (Scintadren™) (Sakar,1976). Both NP-59 and Scintadren™ accumulate in the adrenals slowly,within days, requiring long-lived radionuclides as a label (Iodine-131T_(1/2)=8.04 days; Selen-75 T_(1/2)=120 days). Iodine-131 is alsoemitting beta-radiation, which contritutes considerably to the radiationexposure. The diagnostic use of beta-emitters is no longer state of theart.

In view of the drawbacks of above mentioned agents with respect topatient care (high radiation exposure, repeated imaging procedures), thedevelopment of radiolabeled derivatives of etomidate and metomidatewould greatly improve radionuclide imaging procedures for the detectionand follow-up of adrenal disease.

The invention disclosed herein concerns radiotracers with highselectivity and rapid uptake kinetics, providing metomidate labelledwith a SPECT or PET radionuclide with a short physical half-life.¹²³I-MTO offers optimal imaging characteristics with SPECT, with highaccumulation of the radiotracer in the adrenals, so that imaging may bestarted 10-15 minutes post injection of the radioactive dose. Radiationexposure to the patient is minimized.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 summarizes the relevant characteristics of radioiodinatedmetomidate binding to adrenal membranes.

FIG. 2 provides the IC₅₀ of various etomidate derivatives as inhibitorsof the binding of radioiodinated metomidate.

FIG. 3 describes the accumulation of radioiodinated metomidate indifferent organs in vivo, after up to 120 minutes post-injection.

FIG. 4 provides the target:non-target ratios of radioiodinatedmetomidate after different time intervals post-injection.

FIG. 5 provides the results of experiments describing the in vivodistribution of radiofluorinated etomidate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a compound of formula I,

wherein

-   -   R¹ represents a straight or branched alkyl chain containing from        1 to 4 carbon atoms, wherein the alkyl group is optionally        substituted with a halogen;    -   R² represents a straight alkyl chain in (R)-configuration        containing from 1 to 2 carbon atoms;    -   X denotes a halogen or a radiohalogen;

As used herein, the expression “alkyl,” includes methyl and ethylgroups, and straight-chained or branched propyl groups. Particular alkylgroups are methyl, ethyl, 2-fluoroethyl, n-propyl, and isopropyl,especially methyl and ethyl.

The term “halogen” as used herein, includes iodine, bromine andfluorine, especially iodine.

The compound of formula (I) in accordance with the present invention issuitably a halogenated derivative of formula IA, and a radiolabelledderivative of formula IB:

wherein

-   -   R¹ and R² are as defined above;    -   X denotes a halogen resp. radioiodine;

Preferred are compounds of formula (IB), wherein R¹ and R² is methyl andX is ¹²³I or ¹³¹I, namely(R)-3-[1-(4-[¹²³I]iodophenyl)ethyl-3H-imidazole-4-carboxylic acid methylester (¹²³I-MTO) and(R)-3-[1-(4-[¹³¹I]iodophenyl)ethyl]-3H-imidazole-4-carboxylic acidmethyl ester (¹³¹I-MTO); and

wherein R¹ is ethyl, R² is methyl and X is ¹²³I or ¹³¹I, namely(R)-3-[1-(4-[¹²³I]iodophenyl)ethyl-3H-imidazole-4-carboxylic acid acidethyl ester (¹²³I-ETO) and(R)-3-[1-(4-[¹³¹I]iodophenyl)ethyl]-3H-imidazole-4-carboxylic acid ethylester (131I-ETO).

The present invention includes within its scope stannylated derivativesof formula II:

wherein R¹ and R² are as defined above;

-   -   L represents a trimethyl-, triethyl-, tri-n-propyl- and a        tri-n-butylstannyl group.

The stannylated precursors are prepared from the respective halogenatedcompounds of formula IA to give IIA:

wherein R¹ and R² are as defined above;

-   -   L represents a trimethyl-, triethyl-, tri-n-propyl- and a        tri-n-butylstannyl group.

Compounds of formula II contain a leaving group L, suitably selectedfrom trimethylstannyl, triethylstannyl, tri-n-propylstannyl andtri-n-butylstannyl, especially trimethylstannyl.

Compounds of formula IIA are key intermediates for radiotracer synthesisby oxidative radiohalogenido destannylation. The trialkylstannyl groupsin an aromatic ring can be replaced easily by radiohalogen to yield therespective radioactively labelled compounds of formula (IB).

The invention concerns a process of synthesizing compounds of formula(I) by a stereoselective and regioselective new approach.

The compounds according to the invention may be prepared by a processwhich comprises coupling a compound of formula III:

wherein

-   -   R² represents a straight alkyl chain in (S)-configuration        containing from 1 to 2 carbon atoms;    -   X represents a halogen;    -   with a compound of formula IV:

-   -    wherein    -   R¹ represents a straight or branched alkyl chain containing from        1 to 4 carbon atoms, wherein the alkyl group is optionally        substituted with a halogen;

The reaction between compounds III and IV proceeds with inversion ofconfiguration, to give compounds of formula I:

wherein

-   -   R¹ represents a straight or branched alkyl chain containing from        1 to 4 carbon atoms;    -   R² represents a straight alkyl chain in (R)-configuration        containing from 1 to 2 carbon atoms;    -   X represents a halogen;

Particular alkyl groups are methyl, ethyl, 2-fluoroethyl, n-propyl, andisopropyl, especially methyl and ethyl.

The reaction between compounds III and IV is based on the knownMitsunobu reaction (Mitsunobu, 1981). The alcohol(S)-1-(4-iodophenyl)ethanol (III) is reacted with methylimidazole-4-carboxylate (IV) in the presence of triphenylphosphane and adialkyl azodicarboxylate (preferably di-t-butyl azodicarboxylate).Triphenyl phosphinoxide and the hydrazo ester are by-products of thereaction. The reaction conditions favour activation of the alcohol togenerate the reactive alkoxyphosphonium salt. Methylimidazole-4-carboxylate is expected to be deprotonated, thus N-1 and N-3could react as nucleophile with the alkoxyphosphonium cation to give amixture of two isomeric N-substituted imidazoles. Yet, when coupling IIIand IV at low temperature, alkylation is observed exclusively at N-3with clean inversion.

Previous attempts to react secondary alcohols (alkylaryl or diarylcarbinols) with symmetrical imidazole-4,5-dinitrile produced partialresp. complete racemization by the Mitsunobu reaction (Botta et al.,1994; Corelli et al., 1995).

(S)-III is a key intermediate and needs to be synthesized; methylimidazole-4-carboxylate IV is commercially available.

Since not commercially available, the starting material of formula IIIis prepared by a novel synthetic approach, described hereafter and inscheme 1:

Starting from 4-iodophenyl methyl ketone, which is reduced to theracemic alcohol and converted to (±)-1-(4-iodophenyl)ethylchloroacetate, the racemic ester is subjected to stereoselective enzymehydrolysis. The remaining (S)-isomer of the ester is separated from the(R)-alcohol and transesterified to give (S)-1-(4-iodophenyl)ethanol(ee >98%) III.

Laumen & Schneider (1988) reported that lipase SAM II hydrolysesacetates and chloroacetates of secondary benzyl alcohols with highenantioselectivity, therefore, Schneider's procedure is applied to theresolution of racemic 1-(4-iodophenyl)ethanol. Lipase SAM II is known tohydrolyse preferentially the (R)-esters of secondary benzyl alcohols;however, application with the substrate described in the invention isnew.

(R)-4-Iodo-metomidate is derived exclusively from the (S)-alcohol of theester which is not accepted as substrate by lipase SAM II. Coupling of(S)-1-(4-iodophenyl)ethanol with methyl imidazole-4-carboxylate yields(R)-4-iodo-metomidate with clean inversion.

This novel reaction offers a versatile approach to the synthesis ofcompounds described by formula I. (S)-1-(4-iodophenyl)ethanol (ee >98%)III is synthesized by lipase-catalysed resolution and coupled withmethyl 4-imidazole carboxylate IV. The two fragments are joinedregioselectively at N-3 with clean inversion of configuration producing(R)-methyl 3-[1-(4-iodophenyl)ethyl]-3H-imidazole-4-carboxylate(4-iodo-MTO) IA. Finally, 4-iodo-metomidate is transformed to the4-trimethylstannyl derivative IIA to serve as a precursor for labellingETO and MTO with radiohalogen IB.

Compounds of formula II wherein L represents a leaving group, may beprepared by standard stannylation techniques. The exchange of X (iodine)for the trialkylstannyl substituent (L) is catalysed by tetrakis(triphenylphosphane) palladium to give a stannylated compound of formulaIIA.

Radiohalogenated compounds of formula IB are conveniently prepared byreacting a compound of formula IIA with radiohalogen (Iodine-123;iodine-131) in the presence of an oxidizing agent, at room temperature.

The radioligand ¹³¹I-MTO is produced with a specific activity of 1MBq/nmol, resp. 27.3 μCi/nmol.

The following Examples illustrate the preparation of compounds accordingto the invention.

The compounds in accordance with the present invention potently andselectively bind to adrenocortical membranes (cytochrome P-450β-hydroxylase).

Radioligand Binding

Whole adrenals from adult male Wistar rats were homogenized in 10 mMK₂HPO₄/10 mM HEPES (pH 7.1) with a glass/teflon piston (Potter-type).The homogenate was centrifuged at 35.000×g for 10 min, the pellet wasresuspended in fresh buffer and centrifuged again. Membranes were washed2 more times and stored as aliquots at −80° C.

[¹³¹I]MTO Binding Procedure

Glass vials containing 0.5 ml of 10 mM K₂HPO₄10 mM HEPES, 150 mM NaCl(pH 7.1), 20.000-40.000 cpm [¹³¹I]MTO together with 2 nM 4-iodo-MTO ascarrier (i.e. 1 pmol/vial, resulting in a specific activity of 9-18Ci/mmol), and the adrenal membrane suspension corresponding to 0.2 mgtissue/vial, were immersed in a 23° C. water bath for 20-30 min. Afterincubation, membranes with bound radioligand were isolated by filtrationthrough Whatman GF/B glass fiber filters (presoaked in buffer), followedby 2×4 mL washings with buffer, then filters were measured in agamma-spectrometer.

The association time course of [¹³¹I]MTO to rat adrenal membranes wasevaluated at three ligand concentrations, namely 2 nM, 4 nM, and 11 nMof 4-iodo-MTO, respectively (data pooled from 7 experiments).Computerized curve fitting to the association function B(t)=B₀.[1+exp(−v.t)] resulted in observable association rate constants ofv=0.65 min⁻¹, 1.23 min⁻¹, and 1.34 min⁻¹ corresponding to associationhalf-times from 30 to 60 seconds.

For dissociation experiments, membranes were fully equilibrated with 2nM radioligand, and the dissociation was initiated by the addition ofexcess unlabelled 4-iodo-MTO and stepwise filtration at 15 secintervals. Four individual experiments demonstrated fast reversibilityof binding and resulted in a dissociation constant of b=0.33±0.05 min⁻¹and derived dissociation half-times from 1.8 to 2.4 min.

For saturation studies 2-100 nM 4-iodo-MTO carrier was used. See summaryin FIG. 1. Saturation analysis of [¹³¹I]MTO binding to rat adrenalmembranes (4 representative experiments) produced 15 linearEadie-Hofstee plots suggesting a single binding site. K_(D)=7.4±2.8 nM(15); B_(max)=2.4±0.4 pmol/mg tissue (15). Incubation for 30 minutesusing 10 mM K₂HPO₄/10 mM HEPES, 150 mM NaCl (pH 7.1). 0.2 mg tissue bind˜10% of the free ligand (0.1 pmol).

Displacement of [¹³¹I]MTO Binding (FIG. 2)

Compounds of formula I and derivatives were evaluated as competitiveinhibitors of [¹³¹I]MTO binding. Test compounds were incubated at0.01-100 nM concentrations. Non-specific binding was determined withetomidate (10 μM). The reaction was initiated by the addition ofmembrane and was terminated by filtration through Whatman GF/B filters(presoaked in buffer), followed by 2×4 mL washings with buffer. Thefilters containing membranes with bound radioligand were measured in agamma-spectrometer.

IC₅₀ values (the molar concentration of compound necessary to inhibitbinding by 50%) were determined for each test compound by non-linear,least squares regression analysis, using an iterative curve fittingroutine.

The IC₅₀ values for binding to the cytochrome P-450_(11β) enzyme systemobtained for derivatives of etomidate resp. metomidate of theaccompanying Examples were below 10 nM in each case. Moreover, it wasdemonstrated that (R)-configuration of the methyl substituent at thechiral C-atom is essential for binding, (S)-configuration is nottolerated (IC₅₀=492 nM); cleavage of the ester results in deactivation,the free acid (ETO-acid) is inactive (IC₅₀=123 μM); modification of theester by 2-fluoroethyl (FETO) is tolerated without a loss of affinity(IC₅₀=3.0 nM); the ethyl ester (etomidate) shows the highest potency(IC₅₀=1.1 nM); the methyl ester (metomidate; IC₅₀=3.7 nM) and 4-iodo-MTO(IC₅₀=9.0 nM) show similar potencies. Metyrapone, a known, clinicallyused inhibitor, showed micromolar potency (IC₅₀=1.2 μM) when tested inthis assay.

In Vivo Evaluation of ¹³¹I-MTO (FIGS. 3, 4)

Method: ¹³¹-MTO was used with a radiochemical purity >99% and a specificactivity of 57 GBq/μmol. The radiotracer (0.51.1 MBq; 10-20 μCi) wasinjected into the tail vein of rats (female, 180-220 gram). Groups offour rats were sacrifized at specified times up to 24 hours postinjection. The organs were excised and weighed, the radioactivity wasmeasured at constant geometry using a gamma-spectrometer with aNal(TI)-crystal. The data were expressed as percent of injected dose(ID) per organ and as percent of ID per gram tissue.

Results: ¹³¹-MTO showed high specific uptake in the adrenals ofapproximately 10% ID/g tissue with a radioactivity plateau for 2 hours.The radiotracer is eliminated both by hepatobiliary and by renalexcretion. Renal activity is attributed to ¹³¹I-ETO-acid, which resultsfrom enzymatic cleavage of the methyl ester. The renal activity isincreasing up to 4 hours post injection. Based on calculations of thetarget-to-non-target-ratios the highest contrast for imaging of theadrenals is observed up to one hour post injection.

Thus, [¹³¹I]-I-MTO shows a high potential as radiotracer for thefunctional imaging of adrenal pathology.

Similar distribution kinetics are demonstrated with ¹⁸F-fluoroetomidate.FIG. 5.

The present invention is described below in more detail in connectionwith the synthesis of the radiotracer(R)-3-[1-(4-[¹³¹I]iodophenyl)ethyl]-3H-imidazole-4-carboxylic acidmethyl ester (¹³¹I-MTO); The example is given merely for illustrativepurposes and shall in no way be understood as a limitation of the scopeof the present invention which is given by the patent claims.

EXAMPLES Example 1 Preparation of(R)-3-[1-(4-[¹³¹I]iodophenyl)ethyl]-3H-imidazole-4-carboxylic acidmethyl ester i.e. (¹³¹I-MTO) a. Preparation of(S)-1-(4-iodophenyl)ethanol (III)

The substituted (S)-alcohol was prepared according to scheme 1

Preparation of (±)-1-(4-Iodophenyl)ethanol

A solution of DIBAH (16.45 cm³, 24.67 mmol, 1.5 M solution in toluene)was added dropwise to a stirred mixture of 4-iodoacetophenone (5.08 g,20.65 mmol) in dry diethyl ether (50 cm³) at −78° C. in an atmosphere ofargon. After stirring for 2 h at −78° C., methanol (2 cm³) was added andstirring was continued for 30 min at room temperature before water (10cm³) was added cautiously. 30 min later, the aluminum hydroxide formedwas dissolved in hydrochloric acid (50 cm³, 2 M) under cooling with ice.The organic phase was separated, washed with water and a saturatedaqueous solution of sodium hydrogen carbonate, dried (magnesiumsulphate) and concentrated under reduced pressure. The crude product waspurified by flash chromatography (hexane-dichloromethane 1:2;R_(f)=0.20) on silica gel and bulb to bulb distillation (0.2 mmHg/90-95°C.) to give (±)-(3.91 g, 86%) as liquid which crystallised; mp 47-49° C.(lit. 50.5-51.5° C.).

δ_(H) (400.13 MHz, CDCl₃) 1.40 (3 H, d, J 6.5, CH₃CH), 1.75 (1 H, br s,OH), 4.78 (1 H, q, J 6.5, CH₃CH), 7.06 (2 H, d, J 8.5, 2×H_(arom)), 7.60(2 H, d, J 8.5, 2×H_(arom)); δ_(c)(100.61 MHz, CDCl₃) 25.62 (CH₃CH),70.25 (CH₃CH), 93.09 (IC_(arom)), 127.79 (2 C, 2×HC_(arom)), 137.94 (2C, 2×HC_(arom)), 145.86 (C_(arom)).

Preparation of (±)-1-(4-Iodophenyl)ethyl chloroacetate

Dry pyridine (6.0 cm³) and chloroacetic anhydride (6.2 g, 36.26 mmol)were added to a stirred solution of (±)-1-(4-Iodophenyl)ethanol (5.95 g,24.0 mmol) in dry dichloromethane (100 cm³) at 0° C. under an atmosphereof argon. When the reaction was finished (2 h, TLC:hexane-dichloromethane 3:2, Rf=0.35 for ester), water (40 cm³) andconcentrated hydrochloric acid (3.6 cm³) were added. After stirring for10 min, the organic phase was separated and the aqueous phase wasextracted with dichloromethane (3×15 cm³). The combined organic phaseswere washed with water (50 cm³) and a saturated solution of sodiumhydrogen carbonate (25 cm³), dried (sodium sulfate) and evaporated underreduced pressure. The residue was purified by flash chromatography(hexane-dichloromethane 3:2, Rf=0.35) on silica gel and bulb to bulbdistillation (0.1 mmHg/105° C.) to give chloroacetate (7.09 g, 91%) as acolourless liquid (Found: C, 36.9; H, 3.25, C₁₀H₁₀CllO₂ requires C,37.0; H, 3.1%), which crystallised spontaneously; mp 50-51° C. ν_(max)(Si, film)/cm⁻¹ 2983, 1756, 1591, 1488, 1285, 1176, 1063, 1007; δH(400.13 MHz, CDCl₃) 1.54 (3 H, d, J 7.0, CH₃CH), 4.03 (2 H, d, J 14.8,CH₂Cl), 5.90 (1 H, q, J 7.0, CH₃CH), 7.08 (2 H, d, J 8.5, 2×H_(arom)),7.68 (2 H, d, J 8.0, 2×H_(arom)); δ_(c) (100.61 MHz, CDCl₃); 21.84(CH₃CH), 40.99 (CH₂Cl), 73.84 (CH₃CH), 93.85 (IC_(arom)), 128.08 (2 C,2×HC_(arom)), 137.75 (2 C, 2×HC_(arom)), 140.30 (C_(arom)), 166.45 (CO).

b. Enzymatic Hydrolysis of (±)-1-(4-iodophenyl)ethyl chloroacetate

Racemic chloroacetate (0.835 g, 2.57 mmol) t-butyl methyl ether (4 cm³)and phosphate buffer (50 mmol, sterile, 17 cm³) and lipase SAM 11 (96mg) were stirred vigorously at room temperature. The pH was keptconstant at 7.0 by addition of 0.5 N sodium hydroxide using anautotitrator.

98% of the calculated amount of base were consumed in 2.6 hr. Thereaction was stopped after another 14 h (virtually no base was consumedduring the last 9 hr) by bringing the pH to about 2.0 using 2 Nhydrochloric acid. Water (100 cm³) was added and ester and alcohol wereextracted with dichloromethane (3×200 cm³). The combined organic layerswere washed with water and a saturated solution of sodium hydrogencarbonate (50 cm³ each), dried (sodium sulfate), and evaporated underreduced pressure to leave a residue which was purified by flashchromatography (hexane-dichloromethane 3:2 for chloroacetate, Rf=0.35;hexane-dichloromethane 1:2 for alcohol, Rf=0.20) on silica gel to give(S)-chloroacetate (0.367 g, 44%, [α]²⁰D=82.57 (c 2.57 in acetone), ee98% and [α]²⁰D=+35.68 (c 2.04 in acetone) after chemical hydrolysis) asa liquid and (R)-alcohol (0.393 g, 43%, ee 98%; [α]²⁰D=+35.92 (c 1.96 inacetone) before crystallisation from petroleum ether (40-60°C.)-dichloromethane, afterwards [α]²⁰D=+35.95 (c 2.05 in acetone); mp48-49° C.) as a crystalline solid.

Chemical Hydrolysis of (S)-(1-(4-iodophenyl)ethyl chloroacetate

(S)-Chlorocetate (0.340 g, 1.05 mmol) was dissolved in methanol/sodiummethoxide (17 cm³, obtained by dissolving 69 mg of sodium in 30 cm³ ofdry methanol). After 1 hr, water (few drops) was added and solution wasconcentrated under reduced pressure. Water (30 cm³) and dichloromethane(15 cm³) were added. The organic layer was separated and the aqueous onewas extracted with dichloromethane (2×15 cm³). The combined organiclayers were dried (sodium sulfate) and evaporated to leave a residue,which was purified by flash chromatography (hexane-dichloromethane 1:2,Rf=017) to give (S)-alcohol (0.240 g, 92%, ee 98%) as a crystallinesolid.

c. (R)-(+)-Methyl 3-[1-(4-Iodophenyl)ethyl]-3H-imidazole-4-carboxylate

A solution of (S)-alcohol (1.98 g, 7.98 mmol, ee>98%) in dry THF (14.5cm³) was added dropwise to a stirred solution of methyl3H-imidazole-4-carboxylate (1.008 g, 7.98 mmol) and triphenylphosphane(2.503 g, 9.43 mmol) in dry THF (22.0 cm³) in an atmosphere of argon−30° C. Then, a solution of di-t-butyl azodicarboxylate (2.204 g, 9.57mmol) in dry THF (14.5 cm³) were added and the stirred reaction mixturewas allowed to warm up from −30° C. to 0° C. within 2.5 hr. No alcoholcould by detected by TLC (diethyl ether-diisopropylamine 10:1). Thereaction mixture was concentrated under reduced pressure. The residuewas mixed with diethyl ether (36 cm³) and stirred for 2 h. The crystals(triphenylphosphanoxide and hydrazo ester) were collected and washedwith diethyl ether (3×15 cm³). The filtrate was evaporated und reducedpressure to leave a residue, which was purified by flash chromatography(hexanes-diethyl ether-diisopropylamine 50:30:1; TLC: diethylether-diisopropylamine 10:1, Rf=0.44 for iodide, 0.54 for metomidate) onsilica gel to give p-iodometomidate (1.91 g, 67%, ee 99%); [a]²⁰D+76.0(c 1.09 in acetone).

Found: C. 43.8; H.3.7; N, 7.9. C₁₃H₁₃IN₂O₂ requires C, 43.7; H, 3.8; N,7.7, ν_(max) (Si, film)/cm⁻¹ 2981, 2947, 1712, 1487, 1437, 1363, 1220,1134, 1111, 1006; δ_(H)(400.13 MHz, CDCl₃) 1.81 (3 H, d, J 7.5, CH₃CH),3.77 (3 H, s, OCH₃), 6.26 (1 H, q, J 7.5, CH₃CH), 6.88 (2 H, d, J 8.5,2×H_(arom)), 7.63 (2 H, d, J 8.5, 2×H_(arom)), 7.73 (1H, s,H_(hetarom)), 7.75 (1H, d, J 1.0, H_(hetarom)); δ_(c) (100.61 MHz,CDCl₃) 22.09 (CH₃CH), 51.47 (OCH₃), 54.96 (CH₃CH), 93.46 (IC_(arom)),122.29 (2 C, 2×HC_(arom)), 128.02 (2 C, 2×HC_(arom)), 137.94 (C_(arom)),138.40 (HC_(hetarom)), 139.54 (HC_(hetarom)), 141.10 (CCO), 160.58 (CO).

d. (R)-(+)-Methyl3-[1-(4-trimethylstannylphenyl)ethyl]-3H-imidazole-4-carboxylate

Hexamethylditin (0.645 g. 3.2 mmol, 6.5 cm³ of a solution of 1.0 ghexamethylditin in 10 cm³ of dry toluene),tetrakis(triphenylphosphane)palladium (58 mg, 5 mol %) and triethylamine(1.6 cm³, 11.6 mmol) were added to a stirred solution of iodometomidate(0.368 g. 1.03 mmol, ee >98%) in an atmosphere of argon and refluxed(bath temperature 135° C.) for 17 hr. The cooled solution wasconcentrated under reduced pressure and the residue was purified byflash chromatography (hexane-diethyl ether-diisopropylamine 60:30:1;TLC:diethyl ether-diisopropylamine 10:1, R_(f)=0.71 for stannane,R_(f)=0.50 for iodometomidate) on silica gel to give stannane (0.377 g,96%) as a crystalline solid (Found: C, 48.7; H, 5.6 N, 7.1, C₁₆H₂₂N₂O₂Snrequires C, 48.9; H, 5.6; N, 7.1%); mp 77-79° C. (from hexane);[a]²⁰D=+82.09 (c 2.06 in acetone).

ν_(max) (Si, film)/cm⁻¹ 2981, 1715, 1437, 1362, 1218, 1133, 1110, 1049;δ_(H) (400.13 MHz, CDCl₃) 0.25 (9 H, s, (CH₃)₃Sn, ^(117/119)Snsatellites, 2×d, J 53.2 and 55.2), 1.82 (3 H, d, J 7.0, CH₃CH), 3.77 (3H, s, OCH₃), 6.30 (1 H, q, J 7.0, CH₃CH), 7.13 (2 H, d, J 8.0,2×H_(arom), ^(117/119)Sn satellites, d, J 9.0), 7.43 (2 H, d, J 8.0,2×H_(arom), ^(117/119)Sn satellites, d, J 42.7), 7.71 (1 H, s,H_(hetarom)), 7.74 (1 H s, H_(hetarom)); δ_(c) (100.61 MHz, CDCl₃) −9.61(3 C, (CH₃)₃Sn), 22.16 (CH₃CH), 51.40 (OCH₃), 55.30 (CH₃CH), 122.31(C_(arom)), 125.81 (2×C, 2×HC_(arom)), ^(117/119)Sn satellites, d, J45.9), 136.28 (2×C, 2×HC_(arom), ^(117/119)Sn satellites, d, J 36.8),122.29 (2 C, 2×HC_(arom)), 128.02 (2 C, 2×HC_(arom)), 137.94 (C_(arom)),138.23 (HC_(hetarom)), 139.84 (HC_(hetarom)), 141.00 (C_(arom) or CCO),142.24 (CCO or C_(arom)), 160.68 (CO).

e. Preparation of (R)-(+)-Methyl3-[1-(4-[¹³¹I]Iodophenyl)ethyl]-3H-imidazole-4-carboxylate

30 μg 4-Stannan-MTO is reacted with [¹³¹I]iodide in 10-20 μl NaOH(0.05N), 15 μl chloramine-T (1 mg/ml, aqueous), 6 μl hydrochloric acid(1 N) during 1 min at ambient temperature. The reaction is stopped with6 μl NaOH (1 N), to give the ^(I)compound with a radiochemicalyield: >95%, radiochemical purity: >99%, specific activity: 1 MBq/nmol;chemical purity: >95%.

By following the preparation as described above for (R)-methyl3-[1-(4-iodophenyl)ethyl]-3H-imidazole-4-carboxylate (4-iodo-MTO)further compounds according to the invention can be synthesized, such ase.g. (R)-ethyl 3-[1-(4-iodophenyl)ethyl]-3H-imidazole-4-carboxylate(4-iodo-ETO) and their radioactive analogues.

Literature Related to the Prior Art

-   Basmadjian G P, Hetzel K R, Ice R D, Beierwaltes W H (1975)    Synthesis of a new adrenal cortex imaging agent    6□-[¹³¹I]-iodomethyl-19-norcholest-5(10)en-3□-ol (NP-59). J.    Labelled Compd. & Radiopharm. XI: 427-434.-   Sakar S D, Ice R D, Beierwaltes W H, Gill S P, Balanchandran S,    Basmadjian G P (1976) Selenium-75-19-selenocholesterol—a new adrenal    scanning agent with high concentration in the adrenal cortex. J Nucl    Med 17: 212-217.-   W. H. Beierwaltes, D. M. Wieland, R. D. Ice, J. E. Seabold, S. D.    Sarkar, S. P. Gill, and S. T. Mosley: Localization of radiolabeled    enzyme inhibitors in the adrenal gland. J. Nucl. Med., 17(11),    998-1002, 1976.-   W. H. Beierwaltes, D. M. Wieland, S. T. Mosley, D. P. Swanson, S. D.    Sarkar, J. E. Freitas, J. H Thrall, and K. R. Herwig: Imaging the    adrenal glands with radiolabeled inhibitors of enzymes: concise    communication. J. Nucl. Med., 19(2), 200-203, 1978.-   Wu J. L., Wieland D. M., Beierwaltes W. H., Swanson D. P., Brown L.    E.: Radiolabelled enzyme inhibitors—enhanced localization following    enantiomeric purification. J. Labelled Compd. & Radiopharm., XVI(1),    6-9, 1979.-   Wieland D M: Radiolabeled enzyme inhibitors—Adrenocortical enzymes.    In: Receptor-binding radiotracers, Vol. I, 127-146, Ed. W. C.    Eckelman, Chemical Rubber Co. Press, Cleveland, Ohio, 1982.-   Robien W. and Zolle I. (1983) Synthesis of radioiodinated    metyrapone—A potential agent for functional imaging of the adrenal    cortex. Int. J. Appl. Radiat. Isot. 34: 907-914.-   Allolio B, Stuttmann R, Fischer H, Leonhard W, Winkelman W (1983)    Long-term etomidate and adrenocortical suppression. The Lancet ii,    626.-   Zolle, W. Woloszczuk, and R. Höfer: Synthesis and in vitro    evaluation of metyrapone derivatives as potential inhibitors of    11□-hydroxylase activity. In: Radiopharmaceuticals and labelled    compounds, 337-342, IAEA-CN-45/67, Vienna, 1985.-   Vanden Bossche H, Willemsens G, Cools W, Bellens D (1984) Effects of    etomidate on steroid biosynthesis in subcellular fractions of bovine    adrenals. Biochemical Pharmacology: 33(23), 3861-3868.-   Engelhardt D (1994) Steroid biosynthesis inhibitors in Cushing's    syndrome. Clin Investig 72: 481-488.-   Weber M M, Lang J. Abedinpour F, Zeilberger K, Adelmann B,    Engelhardt D (1993) Different inhibitory effect of etomidate and    ketoconazole on the human adrenal steroid biosynthesis. Clin.    Invest. 71: 933-938.-   Yu, J., Zolle, I., Mertens, J., and Rakias, F.: Synthesis of    2-[¹³¹I]-iodophenyl-metyrapone using Cu(I)-assisted nucleophilic    exchange labelling: Study of the reaction conditions. Nucl. Med. &    Biol. 22(2): 257-262 (1995).-   Godefroi, E. F., Janssen, P. A. J., Van der Eycken, C. A. M., Van    Heertum, A. H. M. T., Niemegeers, C. J. E. (1965)    DL-1-(1-Arylalkyl)imidazole-5-caroxylate esters. A novel type of    hypnotic agents. J. Med. Chem. 8: 220-223.

Synthesis of etomidate U.S. Pat. No. 3,354,173 issued Nov. 21, 1967.Expired November 1984.

Reviews:

-   Mitsunobu O. (1981) Synthesis: 1-28.-   Hughes D. L. (1992) Org. Reactions 42: 335-656.-   Hughes D. L. (1996) Org. Prep. Proced. Int. 28: 127-164.    Reference for Iodinations:-   Merkushev E. B. (1988) Synthesis: 923-937.    Reference for Radioiodinations:-   Ali H. and van Lier J. E. (1996) Synthesis: 423-445. Baldwin    Literature for Enzymatic Hydrolysis:-   Laumen K. and Schneider M. P. (1988) J. Chem. Soc. Chem. Comm.:    598-800.    References for Coupling with Mitsunobu:-   Botta M., Summa V., Trapassi G., Monteagudo E., Corelli F. (1994)    Tetrahedron: Asymmetry 5: 181-184.-   Corelli F., Summa V., Brogi A., Monteagudo E., Botta M. (1995) J.    Org. Chem. 60: 2008-2015.    Ref. for Destannylation-   Baldwin R. M., Zea-Ponce Y., Zoghbi S. S., Laruelle M.,    Al-Tikriti M. S., Sybirska E. H., Malison R. T., Neumeyer J. L.,    Milius R. A., Wang S., Stabin M., Smith E. O., Charney D. S.,    Hoffer P. B., and Innis R. B., (1993) Evaluation of the monoamine    uptake site ligand    [¹²³I]methyl-3□-(4-iodophenyl)tropane-2□-carboxylate ([¹²³I]□-CIT)    in non-human primates: pharmacokinetics, biodistribution and SPECT    brain imaging coregistered with MRI. Nucl. Med. Biol. 20: 597-606.    Literature on Incidentalomas-   Siren J E, Haapiainen R K, Huikuri K T, et al. (1993) Incidentalomas    of the adrenal gland: 36 operated patients and review of literature.    World J. Surg. 17: 634-639.-   Reincke M, Fassnacht M, Vath S, Mora P, Allolio B (1996) Adrenal    incidentalomas: A manifestation of the metabolic syndrome. Endocrine    Research 22(4): 757-761.-   Abecasis M, McLoughlin M J, Lange B, Kudlaw J E (1985) Serendipitous    adrenal masses: Prevalence, significance and management. Am. J.    Surg. 149: 783.-   Herrera M F, Grant C S, van Heerden J A, et al. (1991) Incidentally    discovered adrenal tumors: an institutional perspective. Surgery    110: 1014-1021.-   Kloos R T, Gross M D, Francis I R, Korobkin M, Shapiro B (1995)    Incidentally discovered adrenal masses. Endocrin Rev. 16: 460-484.    Literature Related to Toxic Effect of Etomidate When Used as a    Hypnotic-   Drake W M, Perry L A, Hinds C J, Lowe D G, Reznek R H, Besser G    M (1998) Emergency and prolonged use of intravenous etomidate to    control hypercortisolemia in a patient with Cushing's syndrome and    peritonitis. J. Clin. Endocrinol. Metab. 83: 3542-3544.-   Ledingham I, Watt I (1983) Influence of sedation on mortality in    critically ill multiple trauma patients. Lancet, Jun. 4, 1270.    Lit. for 4-Fluoro-etomidate-   De Coster R., Degryse A.-D., Van Dijk P., Ooms L. A. A.,    Lagerweij E. (1987) Comparison of the effects of etomidate and its    fluoro analogue, R 8110, on plasma cortisol, 11□-deoxycortisol,    17□-hydroxyprogesterone and testosterone concentrations in dogs. J.    vet. Pharmacol. Therap. 10: 227-232.

What is claimed is:
 1. A radiohalogen-labelled compound of formula (I)

wherein R¹ is a linear or branched C₁-C₄ alkyl, and is optionallysubstituted with a halogen selected from the group consisting of F, Cl,Br, I; R² denotes an alkyl group containing 1 or 2 carbon atoms; and Xis a radioactive halogen selected from the group consisting of ¹⁸F,⁷⁶Br, ⁸²Br, ¹²³I, ¹²⁴I, ¹³¹I.
 2. The radiohalogen-labelled compound ofclaim 1 having the formula (IA)

wherein R¹, R², and X are as defined above.
 3. The radiohalogen-labelledcompound of claim 2, wherein X is radioactive iodine.
 4. Theradiohalogen-labelled compound of claim 1, wherein R¹ and R² are eachmethyl and X is ¹²³I, and wherein the compound is ¹²³I-metomidate(¹²³I-MTO).
 5. The radiohalogen-labelled compound of claim 1, wherein R¹is ethyl and R² is methyl and X is ¹³¹I, wherein the compound is¹³¹I-etomidate (¹³¹I-ETO).